The
cryptomelane-type octahedral molecular sieve (OMS-2) exhibits
excellent catalytic activity for the oxidation of volatile organic
compounds (VOCs) due to its unique structure, and single-atom catalysts
(SACs) have recently attracted much attention in heterogeneous catalysis.
Herein, we adopted the hydrothermal redox reaction of KMnO4 and HCl aqueous solution at 120 °C for 12 h to first synthesize
the nanotubular OMS-2 support and used the vitamin C and NaBH4 reduction methods to prepare the OMS-2-supported single-atom
Pt and PtNP catalysts for benzene oxidation, respectively.
It was found that the OMS-2-supported single-atom Pt catalyst with
a Pt loading of 0.0383 wt % (0.0383Pt1/OMS-2) exhibited
the best catalytic performance for benzene oxidation (a 90% benzene
conversion was achieved at 189 °C and 20 000 mL g–1 h–1 (space velocity)), which was
associated with its high surface oxygen vacancy density, good low-temperature
reducibility, and strong benzene adsorption ability. It was also shown
that benzene could be dissociated more readily over the supported
single-atom Pt1 catalyst than over the supported PtNP catalyst, and the phenolates or benzoquinone as well as
methyl groups, lipid, and vinyl species were the main intermediates
in the oxidation of benzene over 0.0383Pt1/OMS-2. The results
of this study are useful in developing the high-performance single-atom
catalysts that can be applied for the oxidative removal of VOCs.
Manganese-based catalysts show a tremendous potential in propane oxidation due to their low cost and good catalytic activity but are easily deactivated by SO 2 . In this work, we first synthesized the mesoporous sodium-doped manganese oxide (meso-Na x MnO y ) using the mesoporous silica (KIT-6) as a template after washing with a concentrated NaOH aqueous solution and then prepared the platinum−cobalt bimetallic singleatom (Pt 1 Co 1 /meso-Na x MnO y ) catalyst using the polyvinyl pyrrolidone-protecting one-pot strategy. It was found that the doping of 13.8 wt % Na dramatically enhanced the SO 2 tolerance of meso-Na x MnO y but inhibited its catalytic activity due to the alkali metal poisoning. Among all of the samples for the oxidation of propane, Pt 1 Co 1 /meso-Na x MnO y showed the highest catalytic activity (a propane conversion of 90% was obtained at 282 °C and a space velocity of 30 000 mL g −1 h −1 , and apparent activation energy, specific reaction rate, and turnover frequency at 250 °C were 76.0 kJ mol −1 , 78.9 × 10 −5 mol g Pt −1 h −1 , and 193.7 × 10 −3 s −1 , respectively), which was associated with the Pt and Co double active sites with good dispersion, abundant electron deficiency, outstanding lattice oxygen mobility, good low-temperature reducibility, and large capacity and weak strength of propane adsorption. More importantly, the Pt 1 Co 1 /meso-Na x MnO y sample possessed the excellent resistance to sulfur dioxide. Propane oxidation occurred via different reaction routes over the MnO 2 , meso-Na x MnO y , and Pt 1 Co 1 /meso-Na x MnO y samples, with the Langmuir− Hinshelwood mechanism being dominated. Propane oxidation over Pt 1 Co 1 /meso-Na x MnO y might follow a pathway of propane → acetone and isopropoxide → carboxylate and fatty ether → CO 2 and H 2 O. Pt and/or Co single atoms are highly dispersed on meso-Na x MnO y . SO 2 is preferentially adsorbed at the Na site in Pt 1 Co 1 /meso-Na x MnO y , which protects the active Mn, Pt, and Co from being poisoned by SO 2 , hence making Pt 1 Co 1 /meso-Na x MnO y show good SO 2 resistance in propane oxidation.
Catalytic performance and moisture and sulfur dioxide resistance are important for a catalyst used for the oxidation of volatile organic compounds (VOCs). Supported noble metals are active for VOC oxidation, but they are easily deactivated by water and sulfur dioxide. Hence, it is highly desired to develop a catalyst with high performance and good moisture and sulfur dioxide resistance in the oxidation of VOCs. In this work, we first adopted the hydrothermal method to synthesize a V2O5-TiO2 composite support, and then employed the polyvinyl alcohol (PVA)-protecting NaBH4 reduction strategy to fabricate xPdPty/V2O5-TiO2 catalysts (x and y are the PdPty loading (0.41, 0.46, and 0.49 wt%) and Pt/Pd molar ratio (2.10, 0.85, and 0.44), respectively; the corresponding catalysts are denoted as 0.46PdPt2.10/V2O5-TiO2, 0.41PdPt0.85/V2O5-TiO2, and 0.49PdPt0.44/V2O5-TiO2). Among all the samples, 0.46PdPt2.10/V2O5-TiO2 exhibited the best catalytic activity for toluene oxidation (T50% = 220 °C and T90% = 245 °C at a space velocity of 40,000 mL/(g h), apparent activation energy (Ea) = 45 kJ/mol), specific reaction rate at 230 °C = 98.6 μmol/(gPt s), and turnover frequency (TOFNoble metal) at 230 °C = 142.2 × 10−3 s−1. The good catalytic performance of 0.46PdPt2.10/V2O5-TiO2 was associated with its well-dispersed PdPt2.10 nanoparticles, high adsorbed oxygen species concentration, good redox ability, large toluene adsorption capacity, and strong interaction between PdPty and V2O5-TiO2. No significant changes in toluene conversion were detected when 5.0 vol% H2O or 50 ppm SO2 was introduced to the reaction system. According to the characterization results, we can realize that vanadium is the main site for SO2 adsorption while PdO is the secondary site for SO2 adsorption, which protects the active Pt site from being poisoned by SO2, thus making the 0.46PdPt2.10/V2O5TiO2 catalyst show good sulfur dioxide resistance.
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